File: norm1est.m

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ufsparse 1.2-7
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function est = norm1est (L,U,P,Q)   % 1-norm estimate of inv(A)
n = size (L,1) ;
for k = 1:5
    if (k == 1)
	est = 0 ;
	x = ones (n,1) / n ;
	jold = -1 ;
    else
	j = min (find (abs (x) == norm (x,inf))) ;
	if (j == jold) break, end ;
	x = zeros (n,1) ;
	x (j) = 1 ;
	jold = j ;
    end
    x = Q * (U \ (L \ (P*x))) ;
    est_old = est ;
    est = norm (x,1) ;
    if (k > 1 && est <= est_old) break, end ;
    s = ones (n,1) ;
    s (find (x < 0)) = -1 ;
    x = P' * (L' \ (U' \ (Q'*s))) ;
end